Rewrite of choose mode, both to simplify and tidy the code and to add

some modern features.

Now the common code is in mode-tree.c, which provides an API used by the
three modes now separated into window-{buffer,client,tree}.c. Buffer
mode shows buffers, client mode clients and tree mode a tree of
sessions, windows and panes.

Each mode has a common set of key bindings plus a few that are specific
to the mode. Other changes are:

- each mode has a preview pane: for buffers this is the buffer content
  (very useful), for others it is a preview of the pane;

- items may be sorted in different ways ('O' key);

- multiple items may be tagged and an operation applied to all of them
  (for example, to delete multiple buffers at once);

- in tree mode a command may be run on the selected item (session,
  window, pane) or on tagged items (key ':');

- displayed items may be filtered in tree mode by using a format (this
  is used to implement find-window) (key 'f');

- the custom format (-F) for the display is no longer available;

- shortcut keys change from 0-9, a-z, A-Z which was always a bit weird
  with keys used for other uses to 0-9, M-a to M-z.

Now that the code is simpler, other improvements will come later.

Primary key bindings for each mode are documented under the commands in
the man page (choose-buffer, choose-client, choose-tree).

Parts written by Thomas Adam.

key-bindings.c

@@ -196,9 +196,9 @@ key_bindings_init(void)
 		"bind p previous-window",
 		"bind q display-panes",
 		"bind r refresh-client",
-		"bind s choose-tree",
+		"bind s choose-tree -s",
 		"bind t clock-mode",
-		"bind w choose-window",
+		"bind w choose-tree -w",
 		"bind x confirm-before -p\"kill-pane #P? (y/n)\" kill-pane",
 		"bind z resize-pane -Z",
 		"bind { swap-pane -U",